1. Field of the Invention
The present invention relates to a multi-purpose valve for facilitating cryogen egress from a cryogen vessel. It is particularly applicable to cryogen-containing cryogen vessels used to cool superconducting magnets, such as those employed in nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI) systems.
2. Description of the Prior Art
As is well known to those skilled in the art, cryogen-cooled superconducting magnets are susceptible to quench events. During a quench event, one part of the superconducting wire forming the magnet becomes resistive for some reason. Reasons include rising temperature due to mechanical stress, movement of the wire within the magnetic field of the magnet casing resistive heating of the cladding, imperfect materials leading to resistive heating. The resistive part of the wire heats up and makes adjacent parts of the superconducting wire resistive also. The quench spreads until a larger proportion of the superconductive magnet is resistive. The large currents typically carried by a superconducting magnet continue to flow through the resistive wire, and cryogen is lost by boil-off due to the heat generated within the cryogen vessel. The pressure within the cryogen vessel increases until a safety valve, known as a quench valve and provided for this purpose, opens. Cryogen gas, and usually also some liquid, escapes through the quench valve until the pressure within the cryogen vessel falls to an acceptable level. The quench valve may then be closed. Arrangements may be provided to self-close the quench valve mechanically, for example using gravity or spring-loaded mechanisms; or a control system may operate an electrically-driven valve actuator. Alternatively, the quench valve may latch in an open state until a service technician closes it.